Fig. 2: Experimental setup.
From: Inverse design of high-dimensional quantum optical circuits in a complex medium
a, A high-dimensional spatially entangled two-photon state is generated via Type-II SPDC in a ppKTP crystal. The two photons are spatially separated by a polarizing beamsplitter (PBS) and sent to two parties, namely, Alice and Bob. Alice performs single-outcome projective measurements \({\hat{\Pi}}_{a}^{\mu}\) that measure whether a photon is carrying spatial mode a from modal basis μ. These are performed by a combination of an SLM (SLM3), SMF and a single-photon avalanche photodiode (APD). Bob implements a top-down programmable circuit that is constructed from an MMF placed between two programmable SLMs (SLM1,2). The circuit is used to program a variety of high-dimensional quantum gates and serves as a generalized multioutcome device. The circular inset shows a coincidence image depicting a five-outcome measurement in basis μ = 1 performed with the Fourier gate \({\mathbb{F}}\) at Bob. The image is obtained by scanning a detector across the output of the circuit, conditioned on the measurement of \({\hat{\Pi}}_{a = 0}^{\mu = 1}\) at Alice, and shows a large intensity in mode 0 due to strong spatial-mode correlations. Coincidence detection events between Alice and Bob are registered by time-tagging electronics. b, Charge-coupled device images demonstrating the operation of the Fourier gate \({\mathbb{F}}\) as a multioutcome measurement of classical macro-pixel modes prepared in basis μ = 1 in dimensions d = {2, 3, 5}. Note that although the input modes have the same amplitude for a given d, they are orthogonal in phase (not seen in the intensity images). L, lens; F, filter; HWP, half-wave plate.
